Reflective mask blank for EUV exposure, and reflective mask

ABSTRACT

A reflective mask blank includes a multilayer reflective film, and a pattern film to be partially etched when processing into a mask. The multilayer reflective film and the pattern film are placed on/above a substrate in this order from the substrate side. The pattern film includes an absorber film and a surface reflection enhancing film in this order from the substrate side. The relation of ((n−1) 2 +k 2 ) 1/2 &gt;((n ABS −1) 2 +k ABS   2 ) 1/2 +0.03 is satisfied, n ABS  and k ABS  being a reflective index and an absorption coefficient of the absorber film at a wavelength of 13.53 nm, respectively, and n and k being a reflective index and an absorption coefficient of the surface reflection enhancing film at a wavelength of 13.53 nm, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/953,580 filed Apr. 16, 2018, allowed, and claims priority fromJapanese Patent Application No. 2017-081235 filed on Apr. 17, 2017, theentire subject matter of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a reflective mask blank which is anoriginal plate for manufacturing a mask for EUV (Extreme Ultra Violet)exposure to be used in an exposure process for manufacturing asemiconductor, and a reflective mask in which a mask pattern has beenformed in a pattern film of the reflective mask blank.

Background Art

In the background art, ultraviolet light having a wavelength of 365 to193 nm is used as a light source of an exposure apparatus used formanufacturing a semiconductor. As the wavelength is shorter, theresolution of the exposure apparatus becomes higher. Therefore, EUVlight having a central wavelength of 13.53 nm is promising as a lightsource of a next-generation exposure apparatus.

EUV light is so easily absorbed by most of substances that a dioptricsystem cannot be used as the exposure apparatus. Therefore, a catoptricsystem and a reflective mask are used for EUV exposure.

In such a reflective mask, a multilayer reflective film which reflectsEUV light is formed on a substrate, and an absorber film for absorbingEUV light is patterned on the multilayer reflective film. A protectivefilm for protecting the multilayer reflective film from etching forforming a mask pattern is usually formed between the multilayerreflective film and the absorber film.

A low thermal expansion glass in which a small quantity of titanium hasbeen added to synthetic quartz in order to prevent pattern distortioncaused by thermal expansion during exposure is often used as thesubstrate. A film in which molybdenum films and silicon films have beenlaminated alternately in about 40 cycles is usually used as themultilayer reflective film. A ruthenium-based material having athickness of 1 to 5 nm is usually used as the protective film. Theruthenium-based material is very hardly etched with gas which does notcontain oxygen. Thus, the ruthenium-based material serves as an etchingstopper when processing into the mask. A tantalum-based material isoften used as the absorber film. The absorber film is often arrangedinto a two-layer structure such as a two-layer structure of a tantalumnitride film and a tantalum oxynitride film in order to make it easy toperform pattern defect inspection after the processing into the mask.

EUV light entering the reflective mask from a lighting optical system ofthe exposure apparatus is reflected by a part (opening portion) wherethe absorber film is absent, and is absorbed by a part (non-openingportion) where the absorber film is present. In this manner, a maskpattern is transferred onto a wafer through a reduction projectionoptical system of the exposure apparatus. The EUV light usually entersthe reflective mask from a direction tilted at an angle of 6 degrees.When the absorber film has a large film thickness, a part shadowed bythe absorber film appears so that the mask pattern cannot be transferredreliably onto the wafer. This problem becomes more conspicuous as theline width of the mask pattern is reduced. It is therefore requested tomake the film thickness of the absorber film thinner.

In order to transfer a pattern with high accuracy in EUV exposure, it isnecessary to reduce the reflectance in a non-opening portion to be 2% orless when a binary mask is used. Patent Literature 1 describes that thereflectance oscillates depending on the film thickness of an absorberfilm. FIG. 14 is a schematic sectional view of a reflective mask blankdescribed in Patent Literature 1. In the reflective mask blank 100 shownin FIG. 14, a multilayer reflective film 120, a protective film 130 andan absorber film 140 are formed sequentially on/above a substrate 110.The reflectance of the reflective mask blank 100 in FIG. 14 oscillatesdepending on the film thickness of the absorber film 140. This isbecause interference occurs between reflected light A reflected by themultilayer reflective film 120 and reflected light B reflected by thesurface of the absorber film 140. There is a minimum value in thereflectance oscillating depending on the film thickness of the absorberfilm. It is necessary to set the film thickness of the absorber film soas to make the minimum value not larger than 2%.

Patent Literature 2 discloses a reflective mask blank 200 in which alaminated absorber 240 having low-refractive material films andhigh-refractive material films laminated alternately in a plurality ofcycles is provided as an absorber film, as shown in FIG. 15 . In thereflective mask blank 200 shown in FIG. 15 , a multilayer reflectivefilm 220, a protective film 230 and the laminated absorber 240 areformed sequentially on/above a substrate 210. In this manner, theamplitude of reflected light B reflected by the laminated absorber 240is increased so that the effect of interference with reflected light Areflected by the multilayer reflective film 220 can be also increased.As a result, the film thickness of the absorber film which can reducethe reflectance to be 2% or less can be made thinner than that in PatentLiterature 1.

-   Patent Literature 1: Japanese Patent No. 4780847-   Patent Literature 2: JP-A-2015-8283-   Patent Literature 3: Japanese Patent No. 5282507-   Patent Literature 4: JP-A-2015-142083-   Non-Patent Literature 1: Proceedings of SPIE Vol. 9635 (2015) 96351C

SUMMARY OF THE INVENTION

In the aforementioned background-art reflective mask blank 100 disclosedin Patent Literature 1, it is necessary to make the film thickness ofthe absorber film 140 more than 60 nm. FIG. 16 shows the simulationresult as to reflectance on the assumption that the absorber film has atwo-layer structure of a tantalum nitride film and a tantalum oxynitridefilm (5 nm thick), based on the assumption that, at a wavelength of13.53 nm, reflectance n and absorption coefficient k of tantalum nitrideare (n, k)=(0.947, 0.031), and reflectance n and absorption coefficientk of tantalum oxynitride are (n, k)=(0.959, 0.028). Although reflectanceof 2% can be obtained when the film thickness of the absorber film 140is 54 nm, implementation with the film thickness of 54 nm is notrealistic when a variation of about 1 nm in film thickness during filmformation is taken into consideration. In consideration of the variationin film thickness, reflectance of 2% or less can be obtained when thefilm thickness of the absorber film is around 61 nm.

On the other hand, in the reflective mask blank 200 disclosed in PatentLiterature 2, the amplitude of the reflected light B reflected by thelaminated absorber 240 is larger than those in the background-artreflective mask blank 100. Thus, the reflectance of 2% or less can beachieved even when the film thickness of the absorber film is 60 nm orless.

However, in the reflective mask blank 200 disclosed in Patent Literature2, the absorber film is constituted by the laminated absorber 240 inwhich low-refractive material films and high-refractive material filmshave been laminated alternately in a plurality of cycles. According toNon-Patent Literature 1, there is a problem that a side wall of amultilayer film may be damaged when the multilayer film is etched orcleansed for processing into a mask. There is also concern over the sameproblem in the laminated absorber 240.

An object of the present invention is to provide a reflective mask blankin which reflectance of EUV light can be made 2% or less even when thefilm thickness of a pattern film is reduced and which can be easilyprocessed into a mask, and a reflective mask.

In an aspect of the present invention, the followings are provided.

<1> A reflective mask blank which is a binary reflective mask blankcomprising:

-   -   a multilayer reflective film which reflects EUV light; and    -   a pattern film to be partially etched when processing into a        mask, the multilayer reflective film and the pattern film being        placed on/above a substrate in this order from the substrate        side;    -   wherein the pattern film includes an absorber film which absorbs        EUV light and a surface reflection enhancing film in this order        from the substrate side, and    -   the following relation is satisfied:        ((n−1)² +k ²)^(1/2)>((n _(ABS)−1)² +k _(ABS) ²)^(1/2)+0.03    -   wherein n_(ABS) is a reflective index of the absorber film at a        wavelength of 13.53 nm, k_(ABS) is an absorption coefficient of        the absorber film at a wavelength of 13.53 nm, n is a reflective        index of the surface reflection enhancing film at a wavelength        of 13.53 nm, and k is an absorption coefficient of the surface        reflection enhancing film at a wavelength of 13.53 nm.

<2> The reflective mask blank according to <1>, wherein the refractiveindex n of the surface reflection enhancing film is 0.95 or less.

<3> The reflective mask blank according to <1> or <2>, wherein filmthickness d of the surface reflection enhancing film satisfies thefollowing relation using the refractive index n:13.53 nm/4n×0.5<d<13.53 nm/4n×1.5.

<4> The reflective mask blank according to <3>, wherein the filmthickness d of the surface reflection enhancing film satisfies thefollowing relation:d<1/10×d _(ABS)

-   -   wherein d_(ABS) is a film thickness of the absorber film.

<5> The reflective mask blank according to any one of <1> to <4>,wherein the surface reflection enhancing film is a ruthenium-basedmaterial film containing ruthenium.

<6> The reflective mask blank according to any one of <1> to <5>,wherein a surface reflection assisting film is formed between theabsorber film and the surface reflection enhancing film in the patternfilm, the surface reflection assisting film satisfying the followingrelation:n<n _(ABS) <n _(B)

-   -   wherein n_(B) is a refractive index of the surface reflection        assisting film at a wavelength of 13.53 nm.

<7> The reflective mask blank according to <6>, wherein the refractiveindex no of the surface reflection assisting film at a wavelength of13.53 nm is 0.95 or more.

<8> The reflective mask blank according to <6> or <7>, wherein filmthickness da of the surface reflection assisting film satisfies thefollowing relation using the refractive index n_(B):13.53 nm/4n _(B)×0.5<d _(B)<13.53 nm/4n _(B)×1.5.

<9> The reflective mask blank according to any one of <6> to <8>,wherein the surface reflection assisting film is a silicon-basedmaterial film containing silicon or an aluminum-based material filmcontaining aluminum.

<10> The reflective mask blank according to any one of <1> to <9>,wherein a protective film for protecting the multilayer reflective filmis provided between the multilayer reflective film and the pattern film.

<11> The reflective mask blank according to any one of <1> to <10>,wherein a hard mask film to be removed when processing into a mask isprovided on the pattern film.

<12> The reflective mask blank according to <11>, wherein the hard maskfilm is selected from the group consisting of a tantalum-based materialfilm containing a tantalum-based material, a chromium-based materialfilm containing a chromium-based material, and a silicon-based materialfilm containing a silicon-based material.

<13> A binary reflective mask in which a pattern film of the reflectivemask blank according to any one of <1> to <12> has been patterned.

In the reflective mask blank and the reflective mask in the presentinvention, a pattern film which will be partially etched when processinginto the mask includes an absorber film and a surface reflectionenhancing film in this order from the substrate side. Due to thepresence of the surface reflection enhancing film, the amplitude of EUVlight reflected by the surface of the pattern film is increased so thatthe effect of interference with EUV light reflected by a multilayerreflective film can be also increased. By use of the effect of theinterference, the film thickness of the pattern film with which thereflectance can be set at 2% or less can be made thinner than the filmthickness of an absorber film in the background art.

The reflective mask blank in the present invention can be easilyprocessed into a mask due to its simple film configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a reflective mask blank inEmbodiment 1.

FIG. 2 is a view similar to FIG. 1 , but illustrating reflected light Creflected by an interface between a surface reflection enhancing film 15and an absorber film 14.

FIG. 3 is a graph showing the relationship between film thickness of apattern film (TaN film and Ru film) or an absorber film (TaN film andTaON film) and reflectance in a case where the pattern film has atwo-layer structure of a TaN film and an Ru film (film thickness of 3.82nm) in the reflective mask blank in the present invention and in a casewhere the absorber film has a two-layer structure of a TaN film and aTaON film (film thickness of 5 nm) in a reflective mask blank in PatentLiterature 1.

FIG. 4 is a graph showing the relationship between film thickness of apattern film (TaN film and Ru film) and reflectance in a case where thepattern film has a two-layer structure of a TaN film and an Ru film(film thickness of 3.82 nm, 1.69 nm or 5.08 nm) in the reflective maskblank in the present invention.

FIG. 5 is a graph of complex indices of refraction in metal elements.

FIG. 6 is a graph showing the relationship between film thickness of apattern film (TaN film and Pd film) or an absorber film (TaN film andTaON film) and reflectance in a case where the pattern film has atwo-layer structure of a TaN film and a Pd film (film thickness of 3.82nm) in the reflective mask blank in the present invention and in a casewhere the absorber film has a two-layer structure of a TaN film and aTaON film (film thickness of 5 nm) in the reflective mask blank inPatent Literature 1.

FIG. 7 is a graph showing the relationship between film thickness of apattern film (TaN film and Ni film) or an absorber film (TaN film andTaON film) and reflectance in a case where the pattern film has atwo-layer structure of a TaN film and an Ni film (film thickness of 3.57nm) in the reflective mask blank in the present invention and in a casewhere the absorber film has a two-layer structure of a TaN film and aTaON film (film thickness of 5 nm) in the reflective mask blank inPatent Literature 1.

FIG. 8 is a graph showing the relationship between film thickness of apattern film (TaN film and Cr film) or an absorber film (TaN film andTaON film) and reflectance in a case where the pattern film has atwo-layer structure of a TaN film and a Cr film (film thickness of 3.63nm) in the reflective mask blank in the present invention and in a casewhere the absorber film has a two-layer structure of a TaN film and aTaON film (film thickness of 5 nm) in the reflective mask blank inPatent Literature 1.

FIG. 9 is a schematic sectional view of a reflective mask blank inEmbodiment 2.

FIG. 10 is a graph showing the relationship between film thickness of apattern film (TaN film and Ru film, or TaN film, Al film and Ru film)and reflectance in a case where the pattern film has a two-layerstructure of a TaN film and an Ru film (film thickness of 3.82 nm) inthe reflective mask blank in the present invention and in a case wherethe pattern film has a three-layer structure of a TaN film, an Al film(film thickness of 3.37 nm) and an Ru film (film thickness of 3.82 nm)in the reflective mask blank in the present invention.

FIG. 11 is a graph showing the relationship between film thickness of apattern film (TaN film and Ru film, or TaN film, Si film and Ru film)and reflectance in a case where the pattern film has a two-layerstructure of a TaN film and an Ru film (film thickness of 3.82 nm) inthe reflective mask blank in the present invention and in a case wherethe pattern film has a three-layer structure of a TaN film, an Si film(film thickness of 3.38 nm) and an Ru film (film thickness of 3.82 nm)in the reflective mask blank in the present invention.

FIG. 12 is a schematic sectional view of a reflective mask blank inEmbodiment 3.

FIG. 13 is a graph showing the relationship between film thickness of apattern film (TaN film and Ru film) and a phase shift amount in a casewhere the pattern film has a two-layer structure of a TaN film and an Rufilm (film thickness of 3.82 nm) in the reflective mask blank in thepresent invention.

FIG. 14 is a schematic sectional view of a reflective mask blank inPatent Literature 1.

FIG. 15 is a schematic sectional view of a reflective mask blank inPatent Literature 2.

FIG. 16 is a graph showing the relationship between film thickness of anabsorber film (TaN film and TaON film) and reflectance in a case wherethe absorber film has a two-layer structure of a TaN film and a TaONfilm (film thickness of 5 nm) in the reflective mask blank in PatentLiterature 1.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

Prior to description of Embodiment 1 of the present invention, thereason why reflectance oscillates depending on the film thickness of anabsorber in Patent Literature 1 will be explained. First, the case wherethe absorber film 140 is absent in FIG. 14 is considered. Only thereflected light A reflected by the multilayer reflective film 120 ispresent. When the amplitude of the reflected light A on the surface ofthe protective film 130 is designated as r_(ML), reflectance R of thereflective mask blank 100 is expressed by the following relation (1).R=|r _(ML)|²  (1)When the multilayer reflective film 120 is a multilayer reflective filmwhich is usually used and has 40 layers composed of molybdenum andsilicon, the reflectance R is about 70% at an exposure wavelength λ of13.53 nm.

Next, the case where the absorber film 140 is present is considered.When the absorber film 140 has a refractive index n_(ABS), an absorptioncoefficient k_(ABS), and a film thickness d_(ABS), the reflected light Areflected by the multilayer reflective film 120 has an amplitudeexpressed by the following expression (2) on the surface of the absorberfilm 140 because the reflected light A travels back and forth over theabsorber film 140.r _(ML)exp(4πi(n _(ABS) +ik _(ABS))d _(ABS)/λ)  (2)

Exactly correction of light obliquely incident should be taken intoconsideration, but the influence is cos(6°)=0.995, that is, not higherthan 1%. Therefore, the influence may be neglected.

When the absorber film 140 is present, the reflected light B on thesurface of the absorber film 140 is necessary to be taken intoconsideration. When the reflected light B has amplitude r_(S), thereflectance R of the reflective mask blank 100 is approximatelyexpressed by the following relation (3).R≈|r _(ML)exp(4πi(n _(ABS) +ik _(ABS))d _(ABS)/λ)+r _(S)|²  (3)

Although another multiple reflection is also generated in the absorberfilm 140, the influence thereof is small.

The relation (3) can be rewritten as the following relation (4).

$\begin{matrix}{R \approx {{{r_{ML}}^{2}{\exp( {{- 8}\pi\; k_{ABS}{d_{ABS}/\lambda}} )}} + {r_{S}}^{2} + {2{r_{ML}}{r_{S}}{\exp( {{- 4}\pi\; k_{ABS}{d_{ABS}/\lambda}} )}{\cos( {{4\pi\; n_{ABS}{d_{ABS}/\lambda}} + \Phi} )}}}} & (4)\end{matrix}$

Here, Φ designates a phase difference between r_(ML) and r_(S). Thethird term of the relation (4) designates an interference term betweenthe reflected light A reflected by the multilayer reflective film 120and the reflected light B on the surface of the absorber film 140. Dueto the influence of this interference, the reflectance R oscillatesdepending on the film thickness d_(ABS) of the absorber.

In Patent Literature 1, the film thickness is set so as to minimize thereflectance by use of the aforementioned interference phenomenon. In acase where the absorber film has a two-layer structure of a tantalumnitride (TaN) film and a tantalum oxynitride (TaON) film (5 nm thick),as illustrated in the simulation result of FIG. 16 , the reflectance hasa minimum value when the film thickness of the absorber film is around61 nm. Thus, the reflectance of 2% or less can be obtained.

As is understood from the relation (4), the interference term isproportional to the absolute value |r_(S)| of the amplitude of thereflected light B reflected by the surface of the absorber film.Therefore, as the absolute value of the amplitude of the reflected lightB is increased, the interference effect is increased so that the minimumvalue of the reflectance R can be reduced accordingly.

The amplitude r_(ABS) of the reflected light B on the absorber surfaceis expressed by the following relation (5) when the absorber film has arefractive index n_(ABS) and an absorption coefficient k_(ABS).r _(ABS)=(n _(ABS) +ik _(ABS)−1)/(n _(ABS) +ik _(ABS)+1)  (5)

Therefore, the absolute value of the amplitude r_(ABS) is expressed bythe following relation (6).|r _(ABS)|=((n _(ABS)−1)² +k _(ABS) ²)^(1/2)/((n _(ABS)+1)² +k _(ABS)²)^(1/2)  (6)

In order to increase the absolute value |r_(ABS)| of the amplitude ofthe reflected light B on the surface of the absorber film, it is desiredto select an absorber material whose refractive index n_(ABS) is assmall as possible. However, various conditions such as opticalconstants, film stress, an etching rate, cleaning resistance, defecttest correspondence, defect correction correspondence, etc. are imposedon the selection of the absorber material. Various problems must besolved when the absorber material is changed from currently usedtantalum-based material to another material.

FIG. 1 shows a reflective mask blank of Embodiment 1 in the presentinvention. In the reflective mask blank 10 shown in FIG. 1 , amultilayer reflective film 12, a protective film 13, and a pattern film16 which will be partially etched when processing into a mask, areformed sequentially on/above a substrate 11. The pattern film 16includes an absorber film 14 and a surface reflection enhancing film 15in this order from the substrate side. Since the mask pattern film 16includes the absorber film 14 and the surface reflection enhancing film15 in this order from the substrate side in the reflective mask blank 10shown in FIG. 1 , interference occurs between reflected light Areflected by the multilayer film 12 and reflected light B reflected bythe surface of the surface reflection enhancing film 15 forming thesurface of the mask pattern film 16. Due to the interference, thesurface reflected light can be enhanced while the selection of theabsorber film 14 can be made easier.

In the reflective mask blank 10 shown in FIG. 1 , the absolute value ofthe amplitude of the reflected light B reflected by the surface of thesurface reflection enhancing film 15 is expressed by the followingrelation (7) when the surface reflection enhancing film 15 has arefractive index n and an absorption coefficient k.|r|=((n−1)² +k ²)^(1/2)/((n+1)² +k ²)^(1/2)  (7)

In the reflective mask blank 10 shown in FIG. 1 , when the absolutevalue |r| of the amplitude of the reflected light B reflected by thesurface of the surface reflection enhancing film 15 is made larger thanthe aforementioned absolute value |r_(ABS)| of the amplitude of thereflected light B on the surface of the absorber film, the effect of theinterference can be increased to reduce the minimum value of thereflectance R of the reflective mask blank 10.

The reflective mask blank 10 shown in FIG. 1 satisfies the followingrelation (8) when the absorber film 14 has a refractive index n_(ABS)and an absorption coefficient k_(ABS) at a wavelength of 13.53 nm andthe surface reflection enhancing film 15 has a refractive index n and anabsorption coefficient k at the same wavelength.((n−1)² +k ²)^(1/2)>((n _(ABS)−1)² +k _(ABS) ²)^(1/2)+0.03  (8)

The aforementioned relation (8) is derived from the aforementionedrelations (6) and (7). In the wavelength range of EUV light, therefractive index n_(ABS) of the absorber film 14 and the refractiveindex n of the surface reflection enhancing film 15 take values close to1, and the absorption coefficient k_(ABS) of the absorption film 14 andthe absorption coefficient k of the surface reflection enhancing film 15take values close to 0. Therefore, the denominator in each of therelations (6) and (7) is approximately 2 regardless of the constituentmaterials of the absorber film 14 and the surface reflection enhancingfilm 15. The constant 0.03 in the right side of the relation (8) is avalue which was found out from the graph of complex indices ofrefraction in metal elements shown in FIG. 5 when the constituentmaterial of the surface reflection enhancing film 15 and the constituentmaterial of the absorber film 14 were specified.

When the aforementioned relation (8) is satisfied, the effect of theinterference can be increased to reduce the minimum value of thereflectance R of the reflective mask blank 10.

When the refractive index n of the surface reflection enhancing film 15is selected to be smaller than the refractive index n_(ABS) of theabsorber film 14, the absolute value of the amplitude of the reflectedlight B is larger. When the absorber film 14 is made of a tantalum-basedmaterial in the same manner as in the background art, it is desired thatthe refractive index n of the surface refection enhancing film 15 is0.95 or less.

In addition, as shown in FIG. 2 , reflected light C is also generatedfrom the interface between the surface reflection enhancing film 15 andthe absorber film 14 in the reflective mask blank 10. When phases of thereflected lights B and C are aligned, the surface reflected light can befurther enhanced due to interference occurring between the reflectedlight A and each of the reflected lights B and C.

Conditions with which the phases of the reflected light B and thereflected light C can be aligned are provided by the following relation(9) using the refractive index n and film thickness d of the surfacereflection enhancing film 15. The resultant come to be the optimum valueof the film thickness d of the surface reflection enhancing film 15.d=λ/4n  (9)

FIG. 3 shows the simulation result as to the dependency of reflectanceon the film thickness when a tantalum nitride (TaN) film was selected asthe absorber film 14 and a ruthenium (Ru) film was selected as thesurface reflection enhancing film 15. Values (n, k)=(0.886, 0.017) at awavelength of 13.53 nm were used as the refractive index n and theabsorption coefficient k of the ruthenium film.

In addition, the film thickness of the ruthenium film was set at 13.53nm/4/0.886=3.82 nm based on the aforementioned relation (9).

In addition, the film thickness of the pattern film 16 which was a totalvalue of the film thicknesses of the absorber film 14 and the surfacereflection enhancing film 15 was selected in the abscissa in FIG. 3 .

In FIG. 3 , the simulation result of FIG. 16 , that is, the simulationresult as to the case of the reflective mask blank in Patent Literature1 is shown together.

From comparison between the two simulation results, it can be found thatthe amplitude of the reflectance in the reflective mask blank in thepresent invention is larger. This is because the surface reflection isenhanced due to the refractive index of ruthenium which is smaller thanthe refractive index of the tantalum-based material. Although theabsorber film thickness must be set at about 61 nm in the background artin order to set the reflectance at 2% or less, the pattern filmthickness can be reduced to about 48 nm in the present invention.

FIG. 4 shows the simulation result as to the dependency of reflectanceon the film thickness when a tantalum nitride (TaN) film was selected asthe absorber film 14 and a ruthenium (Ru) film was selected as thesurface reflection enhancing film 15 in the same manner as in FIG. 3 .However, the Ru film had three film thicknesses, more specifically, 1.69nm, 3.82 nm and 5.08 nm, in FIG. 4 . The amplitude of the reflectance inthe case where the film thickness of the Ru film was 1.69 nm or 5.08 nmwas smaller than the case where the film thickness of the Ru film wasoptimized to be 3.82 nm.

The preferred value of the film thickness d of the surface reflectionenhancing film 15 is expressed by the following relation (10) from thesimulation result.13.53 nm/4n×0.5<d<13.53 nm/4n×1.5  (10)

As apparent from above, the ruthenium (Ru) film has suitable propertiesas the surface reflection enhancing film 15. However, Ru is used as theprotective film 13 of the multilayer reflective film 12. The Ru film ishardly etched in comparison with the tantalum-based material forming theabsorber film 14. When the surface reflection enhancing film 15 is an Rufilm, it is therefore preferable that the film thickness of the surfacereflection enhancing film 15 is reduced to improve processability into amask. When the surface reflection enhancing film 15 is an Ru film, it ispreferable that the film thickness d of the surface reflection enhancingfilm 15 satisfies the following relation (11) when the absorber film 14has a film thickness d_(ABS).d<1/10×d _(ABS)  (11)

Although description has been made above in the case where the surfacereflection enhancing film 15 is an Ru film, the constituent material ofthe surface reflection enhancing film 15 is not limited thereto. Anymaterial may be used as long as it has a refractive index n and anabsorption coefficient k satisfying the aforementioned relation (8) inrelation to the refractive index n_(ABS) and absorption coefficientk_(ABS) of the absorber film 14.

FIG. 5 shows the graph of complex indices of refraction in metalelements. The broken line in FIG. 5 corresponds to the followingrelation (12).((n−1)² +k ²)^(1/2)=((n _(ABS)−1)² +k _(ABS) ²)^(1/2)+0.03  (12)

When the constituent material of the absorber film 14 is a metal elementon the right side of the broken line in FIG. 5 , a metal element on theleft side of the broken line can be used as the constituent material ofthe surface reflection enhancing film 15. For example, when theconstituent material of the absorber film 14 is a tantalum-basedmaterial such as TaN or TaON, Ag, Pt, Pd, Au, Ru or Ni can be used asthe constituent material of the surface reflection enhancing film 15.

When the constituent material of the surface reflection enhancing film15 is the other material than the aforementioned Ru, the preferred valueof the film thickness d of the surface reflection enhancing film 15 canbe expressed by the aforementioned relation (10). In addition, when theconstituent material of the surface refection enhancing film 15 is moredifficult to be etched than the tantalum-based material, it ispreferable that the film thickness d of the surface reflection enhancingfilm 15 satisfies the aforementioned relation (11).

FIG. 6 shows the simulation result as to the dependency of reflectanceon the film thickness when a tantalum nitride (TaN) film was selected asthe absorber film 14 and a palladium (Pd) film was selected as thesurface reflection enhancing film 15. Palladium (Pd) is located on theleft side of the broken line in FIG. 5 . Values (n, k)=(0.876, 0.046) ata wavelength of 13.53 nm were used as the refractive index n andabsorption coefficient k of the palladium film.

In addition, the film thickness of the palladium film was set at 13.53nm/4/0.876=3.86 nm based on the aforementioned relation (9).

In addition, the film thickness of the pattern film 16 which was a totalvalue of the film thicknesses of the absorber film 14 and the surfacereflection enhancing film 15 was selected in the abscissa in FIG. 6 .

In FIG. 6 , the simulation result of FIG. 16 , that is, the simulationresult as to the case of the reflective mask blank in Patent Literature1 is shown together.

From comparison between the two simulation results, it can be found thatthe amplitude of the reflectance in the reflective mask blank in thepresent invention is larger. This is because the surface reflection isenhanced due to the refractive index of palladium which is smaller thanthe refractive index of the tantalum-based material. Although theabsorber film thickness must be set at about 61 nm in the background anin order to set the reflectance at 2% or less, the pattern filmthickness can be reduced to about 40 nm in the present invention.

FIG. 7 shows the simulation result as to the dependency of reflectanceon the film thickness when a tantalum nitride (TaN) film was selected asthe absorber film 14 and a nickel (Ni) film was selected as the surfacereflection enhancing film 15. Nickel (Ni) is located on the left side ofthe broken line in FIG. 5 . Values (n, k)=(0.948, 0.073) at a wavelengthof 13.53 nm were used as the refractive index n and absorptioncoefficient k of the nickel film.

In addition, the film thickness of the nickel film was set at 13.53nm/4/0.948=3.57 nm based on the aforementioned relation (9).

In addition, the film thickness of the pattern film 16 which was a totalvalue of the film thicknesses of the absorber film 14 and the surfacereflection enhancing film 15 was selected in the abscissa in FIG. 7 .

In FIG. 7 , the simulation result of FIG. 16 , that is, the simulationresult as to the case of the reflective mask blank in Patent Literature1 is shown together.

From comparison between the two simulation results, it can be found thatthe amplitude of the reflectance in the reflective mask blank in thepresent invention is larger. This is because the surface reflection isenhanced due to the refractive index of nickel which is smaller than therefractive index of the tantalum-based material. Although the absorberfilm thickness must be set at about 61 nm in the background art in orderto set the reflectance at 2% or less, the pattern film thickness can bereduced to about 46 nm in the present invention.

FIG. 8 shows the simulation result as to the dependency of reflectanceon the film thickness when a tantalum nitride (TaN) film was selected asthe absorber film 14 and a chromium (Cr) film was selected as thesurface reflection enhancing film 15. Chromium (Cr) is located on theright side of the broken line in FIG. 5 . Values (n, k)=(0.932, 0.039)at a wavelength of 13.53 nm were used as the refractive index n andabsorption coefficient k of the chromium film.

In addition, the film thickness of the chromium film was set at 13.53nm/4/0.932=3.63 nm based on the aforementioned relation (9).

In addition, the film thickness of the pattern film 16 which was a totalvalue of the film thicknesses of the absorber film 14 and the surfacereflection enhancing film 15 was selected in the abscissa in FIG. 8 .

In FIG. 8 , the simulation result of FIG. 16 , that is, the simulationresult as to the case of the reflective mask blank in Patent Literature1 is shown together.

From comparison between the two simulation results, it can be found thatthere is a small difference in amplitude of reflectance. This is becausethe difference in refractive index between chromium and thetantalum-based material is too small not to enhance the surfacereflection. In order to set the reflectance at 2% or less, the filmthickness can be reduced to about 54 nm.

Embodiment 2

As is described above, the reflective mask blank 10 in Embodiment 1 ofthe present invention has an effect of interference between EUV lightreflected by the pattern film surface and EUV light reflected by themultilayer reflective film due to the presence of the surface reflectionenhancing film 15 satisfying the aforementioned relation (8), so thatthe pattern film thickness with which the reflectance can be set at 2%or less can be made thinner than the absorber film thickness in thebackground art.

FIG. 9 shows a reflective mask blank in Embodiment 2 of the presentinvention. In the reflective mask blank 20 in Embodiment 2 as shown inFIG. 9 , a multilayer reflective film 22, a protective film 23, and apattern film 27 which will be partially etched when processing into amask, are formed sequentially on/above a substrate 21. The pattern film27 includes an absorber film 24, a surface reflection assisting film 26and a surface reflection enhancing film 25 in this order from thesubstrate side. That is, the surface reflection assisting film 26 isformed between the absorber film 24 and the surface reflection enhancingfilm 25 in the pattern film 27.

When the surface reflection assisting film 26 has a refractive indexn_(B) at a wavelength of 13.53 nm, the surface reflection assisting film26 satisfies the following relation (13) in relation to the refractiveindex n_(ABS) of the absorber film 24 at a wavelength of 13.53 nm andthe refractive index n of the surface reflection enhancing film 25 at awavelength of 13.53 nm.n<n _(ABS) <n _(B)  (13)

With such a configuration, the amplitude of the EUV light reflected bythe pattern film surface is further increased, and the effect ofinterference with the EUV light reflected by the multilayer reflectivefilm is also increased. Thus, the pattern film thickness with which thereflectance can be set at 2% or less can be made thinner.

As described above, when the absorber film 24 is made of thetantalum-based material as in the background art, it is desired that therefractive index n of the surface reflection enhancing film 25 is 0.95or less. It is therefore desired that the refractive index no of thesurface reflection assisting film 26 is 0.95 or more.

Although not shown, reflected light reflected by the surface of thesurface reflection enhancing film 25, reflected light reflected by theinterface between the surface reflection enhancing film 25 and thesurface reflection assisting film 26, and reflected light reflected bythe interface between the surface reflection assisting film 26 and theabsorber film 24 are generated in the reflective mask blank 20, as shownin FIG. 9 . When the phases of those reflected lights are aligned, theamplitude of the EUV light reflected by the pattern film surface can befurther increased.

As for conditions with which the phases of the reflected lights can bealigned, when the surface reflection enhancing film 25 has a refractiveindex n and film thickness d and the surface reflection assisting film26 has a refractive index n_(B) and film thickness d_(B), the optimumvalues of d and d_(B) are expressed as follows.d=λ/4n  (14)d=λ/4n _(B)  (15)

FIG. 10 shows the simulation result as to the dependency of reflectanceon the film thickness when a tantalum nitride (TaN) film was selected asthe absorber film 24, an aluminum (Al) film was selected as the surfacereflection assisting film 26, and a ruthenium (Ru) film was selected asthe surface reflection enhancing film 25. Values (n_(B), k_(B))=(1.003,0.03) at a wavelength of 13.53 nm were used as the refractive indexn_(B) and absorption coefficient k_(B) of the aluminum (Al) film. Inaddition, the film thickness of the aluminum film was set at 13.53nm/41.003=3.37 nm based on the aforementioned relation (15).

In addition, the film thickness of the pattern film 27 which was a totalvalue of the film thicknesses of the absorber film 24, the surfacereflection assisting film 26 and the surface reflection enhancing film25 was selected in the abscissa in FIG. 10 .

FIG. 10 also shows the simulation result of FIG. 3 , that is, thesimulation result as to the case of the reflective mask blank (theabsorber film 14 which is a tantalum nitride (TaN) film, and the surfacereflection enhancing film 25 which is a ruthenium (Ru) film) inEmbodiment 1 of the present invention.

From FIG. 10 , it can be found that when the surface reflectionassisting film is formed between the absorber film and the surfacereflection enhancing film, the amplitude of the reflectance is largerthan the case where the surface reflection assisting film is absent. Thepattern film thickness with which the reflectance can beset at 2% orless can be reduced to about 40 nm. From this result, it can be foundthat an aluminum-based material film containing aluminum (Al) ispreferred as the surface reflection assisting film 26.

The preferred value of the film thickness d_(B) of the surfacereflection assisting film 26 is expressed by the following expression(16) based on the simulation result.13.53 nm/4n _(B)×0.5<d _(B)<13.53 nm/4n _(B)×1.5  (16)

Although description is made above in the case where the surfacereflection assisting film 26 is an Al film, the constituent material ofthe surface reflection assisting film 26 is not limited thereto. Anymaterial may be used as long as the refractive index n_(B) at awavelength of 13.53 nm satisfies the aforementioned relation (13) inrelation to the refractive index n_(ABS) of the absorber film 24 and therefractive index n of the surface reflection enhancing film 25.

For example, a metal element on the right side of the constituentmaterial of the absorber film 24 can be used as the constituent materialof the surface reflection assisting film 26. For example, when theconstituent material of the absorber film 24 is a tantalum-basedmaterial such as TaN or TaON, a silicon-based material containingsilicon (Si) can be used as the constituent material of the surfacereflection assisting film 26.

When the constituent material of the surface reflection assisting film26 is the other material than the aforementioned Al, the preferred valueof the film thickness do of the surface reflection assisting film 26 canbe expressed by the aforementioned relation (16).

FIG. 11 shows the simulation result as to the dependency of reflectanceon the film thickness when a tantalum nitride (TaN) film was selected asthe absorber film 24, a silicon (Si) film was selected as the surfacereflection assisting film 26 and a ruthenium (Ru) film was selected asthe surface reflection enhancing film 25. Values (n_(B), k_(B)) (0.999,0.002) at a wavelength of 13.53 nm were used as the refractive indexn_(B) and absorption coefficient k_(B) of the silicon (Si) film.

In addition, the film thickness of the silicon film was set at 13.53nm/4/0.999=3.38 nm from the aforementioned relation (15).

In addition, the film thickness of the pattern film 27 which was a totalvalue of the film thicknesses of the absorber film 24, the surfacereflection assisting film 26 and the surface reflection enhancing film25 was selected in the abscissa in FIG. 11 .

FIG. 11 also shows the simulation result of FIG. 3 , that is, thesimulation result as to the reflective mask blank (the absorber film 14which is a tantalum nitride (TaN) film, and the surface reflectionenhancing film 25 which is a ruthenium (Ru) film) in Embodiment 1 of thepresent invention.

From FIG. 11 , it can be found that when the surface reflectionassisting film is formed between the absorber film and the surfacereflection enhancing film, the amplitude of the reflectance is largerthan the case where the surface reflection assisting film is absent. Thepattern film thickness with which the reflectance can be set at 2% orless can be reduced to about 47 nm.

As a film configuration similar to the reflective mask blank inEmbodiment 2 of the present invention, Patent Literature 2 discloses asan absorber film, the reflective mask blank 20 including the laminatedabsorber 240 in which low-refractive material films and high-refractivematerial films have been laminated alternately in a plurality of cycles,specifically in at least four cycles, using a structure of onelow-refractive material film and one high-refractive material as onecycle, as shown in FIG. 15 . On the other hand, the reflective maskblank 20 in Embodiment 2 of the present invention uses only one cycle ofa laminated film of a low-refractive material film (surface reflectionenhancing film 25) and a high-refractive material film (surfacereflection assisting film 26) in the pattern film 27. Accordingly, thereflective mask blank 20 in Embodiment 2 of the present invention canreduce damage against the side wall of the pattern film when the patternfilm is etched or cleansed when processing into a mask, in comparisonwith the reflective mask blank 200 in Patent Literature 2.

Embodiment 3

In the reflective mask blank 10 in Embodiment 1 of the presentinvention, the surface reflection enhancing film 15 is provided as alayer above the absorber film 14 in the configuration of the patternfilm 16. A load with which the pattern film 16 is etched is larger thanthe case where only the absorber film 14 is etched as in thebackground-art reflective film blank 100 shown in FIG. 14 . Accordingly,as the minimum line width of the mask pattern is reduced, it is moredifficult to etch the pattern film 16. As a usual solution to thisproblem, there is a method in which a hard mask film made of a materialhaving durability against the etching conditions of the absorber film isadded.

In a reflective mask blank 30 in Embodiment 3 of the present inventionas shown in FIG. 12 , a multilayer reflective film 32, a protective film33, and a pattern film 36 which will be partially etched when processinginto a mask, are formed sequentially on/above a substrate 31. Thepattern film 36 includes an absorber film 34 and a surface reflectionenhancing film 35 in this order from the substrate side. In thereflective mask blank 30 in Embodiment 3 of the present invention asshown in FIG. 12 , a hard mask film 37 is provided on the pattern film36. In the reflective mask blank 30 in Embodiment 3 of the presentinvention, the surface reflection assisting film as in the reflectivemask blank in Embodiment 2 of the present invention may be providedbetween the absorber film 34 and the surface reflection enhancing film35.

The material of the hard mask film 37 must be a material which canensure the selectivity when the surface reflection enhancing film 35 isetched. As the material of the hard mask film 37, a tantalum-basedmaterial, a chromium-based material or a silicon-based material can beselected depending on the material of the surface reflection enhancingfilm 35.

In the above description, a tantalum nitride (TaN) film is selected asthe absorber film 14 and a ruthenium (Ru) film is selected as thesurface reflection enhancing film 15 as an example of the reflectivemask blank 10 in Embodiment 1 of the present invention. When a tantalumnitride (TaN) film is selected as the absorber film 34 and a ruthenium(Ru) film is selected as the surface reflection enhancing film 35 in thereflective mask blank 30 in Embodiment 3 of the present invention, atantalum nitride (TaN) film can be selected as the hard mask film 37. Inthis case, higher etching selectivity with oxygen gas can be achievedwhen the ruthenium (Ru) film as the surface reflection enhancing film 35is etched. After that, the absorber film 34 and the hard mask film 37are etched simultaneously by chlorine gas. Thus, the step of removingthe hard mask can be simplified.

Film configurations formed of a tantalum-based film and a ruthenium filmsimilarly to the present invention are disclosed in Patent Literature 3and Patent Literature 4. However, Patent Literatures 3 and 4 relate tohalftone phase shift masks. The reflectance of EUV light in thereflective mask blank in the present invention is low to be 2% or less,which is much lower than reflectance of 6% usually used in such a halftone phase shift mask. Further, as shown in FIG. 13 , in the reflectivemask blank in the present invention, the phase shift amount for theoptimum pattern film thickness of 48 nm is 142 degrees, which deviateslargely from the optimum value of 180 degrees as a phase shift mask.FIG. 13 is the graph showing the relationship between the film thicknessof the pattern film (TaN film and Ru film) and the phase shift amountwhen the pattern film in the reflective mask blank in the presentinvention has a two-layer structure of a TaN film and an Ru film (filmthickness of 3.82 nm).

In this manner, the reflective mask blank in the present inventiontransfers a mask pattern onto a wafer due to a function of so-calledbinary mask, but does not have a function as a phase shift mask.

In the reflective mask blank in each of Embodiments 1 to 3 of thepresent invention, any other configuration than the pattern film and thehard mask film is similar to that of the background-art reflective maskblank. That is, a low thermal expansion glass in which a small quantityof titanium has been added to synthetic quartz in order to preventpattern distortion caused by thermal expansion during exposure is usedsuitably as the substrate. A film in which molybdenum films and siliconfilms have been laminated alternately in about 40 cycles is usually usedas the multilayer reflective film. A ruthenium-based material having athickness of 1 to 5 nm is usually used as the protective film.

Embodiment 4

Embodiment 4 of the present invention is directed to a binary reflectivemask in which the pattern film is patterned in the reflective mask blankin any one of Embodiments 1 to 3 of the present invention.

Example

The present invention will be described below along its example. In thisexample, the reflective mask blank 10 shown in FIG. 1 is formed.

An SiO₂—TiO₂ based glass substrate (6 inches (about 152 mm) square inouter dimensions, and about 6.3 mm thick) is used as a substrate 11 forforming films thereon. The coefficient of thermal expansion of the glasssubstrate is 0.02×10⁻⁷/° C. The glass substrate is polished to have asmooth surface whose surface roughness (rms) is 0.15 nm or less andwhose flatness is 100 nm or less. A chromium film having a thickness of100 nm is formed on the back surface side of the substrate 11 by use ofa magnetron sputtering method. Thus, a highly dielectric coating havinga sheet resistance of 100Ω/square is applied to the back surface side ofthe substrate 11. Using the chromium film formed, the substrate 11 isfixed to a usual electrostatic chuck having a flat plate shape, andsilicon films and molybdenum films are formed alternately on/above thesurface of the substrate 11 repeatedly in 40 cycles by use of an ionbeam sputtering method. Thus, a multilayer reflective film 12 having atotal film thickness of 272 nm ((4.5 nm+2.3 nm)×40) is formed.

Further, a ruthenium film (film thickness of 2.5 nm) is formed on themultilayer reflective film 12 by use of an ion beam sputtering method.Thus, a protective film 13 is formed.

Next, an absorber film 14 made of tantalum nitride is formed to have athickness of 44 nm on the protective film 13 by magnetron sputtering.Tantalum is used as a target, and mixed gas of argon and nitrogen isused as sputtering gas.

Finally, a surface reflection enhancing film IS made of ruthenium isformed to have a thickness of 3.8 nm on the absorber film 14 bymagnetron sputtering. Ruthenium is used as a target, and argon is usedas sputtering gas.

EUV light having a wavelength of 13.5 nm enters the reflective maskblank 10 manufactured with an incident angle of 6 degrees, in order tomeasure the reflectance. The measured reflectance is 1.3%.

According to the reflective mask blank in this example, the total filmthickness of the pattern film is 47.8 nm, which can be reduced on alarge scale in comparison with the absorber film thickness of 61 nm inthe background-art reflective mask blank.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   -   10 Reflective mask blank    -   11 Substrate    -   12 Multilayer reflective film    -   13 Protective film    -   14 Absorber film    -   15 Surface reflection enhancing film    -   16 Pattern film    -   20 Reflective mask blank    -   21 Substrate    -   22 Multilayer reflective film    -   23 Protective film    -   24 Absorber film    -   25 Surface reflection enhancing film    -   26 Surface refection assisting film    -   27 Pattern film    -   30 Reflective mask blank    -   31 Substrate    -   32 Multilayer reflective film    -   33 Protective film    -   34 Absorber film    -   35 Surface refection enhancing film    -   36 Pattern film    -   37 Hard mask film    -   100 Reflective mask blank    -   110 Substrate    -   120 Multilayer reflective film    -   130 Protective film    -   140 Absorber film    -   200 Reflective mask blank    -   210 Substrate    -   220 Multilayer reflective film    -   230 Protective film    -   240 Laminated absorber film

The invention claimed is:
 1. A reflective mask blank which is a binaryreflective mask blank comprising: a multilayer reflective film whichreflects EUV light; and a pattern film to be partially etched whenprocessing into a mask, the multilayer reflective film and the patternfilm being placed on/above: a substrate in this order from the substrateside; wherein the pattern film includes an absorber film which absorbsEUV light and a surface reflection enhancing film in this order from thesubstrate side, and the following relation is satisfied:((n−1)₂ +k ₂)^(1/2)>(n _(ABS)−1)² +k _(ABS) ²)^(1/2)+0.03 whereinn_(ABS) is a reflective index of the absorber film at a wavelength of13.53 nm, k_(ABS) is an absorption coefficient of the absorber film at awavelength of 13.53 nm, n is a reflective index of the surfacereflection enhancing film at a wavelength of 13.53 nm, and k is anabsorption coefficient of the surface reflection enhancing film at awavelength of 13.53 nm, wherein the surface reflection enhancing film isdirectly laminated on the absorber film, the pattern film has athickness of 54 nm or less, and wherein film thickness d of the surfacereflection enhancing film satisfies the following relation using therefractive index n:13.53 nm/4n×0.5<d<13.53 nm/4n×1.5.
 2. The reflective mask blankaccording to claim 1, wherein the refractive index n of the surfacereflection enhancing film is 0.95 or less.
 3. The reflective mask blankaccording to claim 1, wherein the film thickness d of the surfacereflection enhancing film satisfies the following relation:d<1/10×d _(ABS) wherein d_(ABS) is a film thickness of the absorberfilm.
 4. The reflective mask blank according to claim 1, wherein thesurface reflection enhancing film is a ruthenium-based material filmcontaining ruthenium.
 5. The reflective mask blank according to claim 1,wherein a protective film for protecting the multilayer reflective filmis provided between the multilayer reflective film and the pattern film.6. The reflective mask blank according to claim 1, wherein a hard maskfilm to be removed when processing into a mask is provided on thepattern film.
 7. The reflective mask blank according to claim 6, whereinthe hard mask film is selected from the group consisting of atantalum-based material film containing a tantalum-based material, achromium-based material film containing a chromium-based material, and asilicon-based material film containing a silicon-based material.
 8. Abinary reflective mask in which a pattern film of the reflective maskblank according to claim 1 has been patterned.